This application is based on applications No. 2000-114340 and 2000-145833 filed in Japan, the contents of which are hereby incorporated for reference.
1. Field of the Invention
This invention relates to a hollow rack shaft for a steering apparatus used in automobiles.
2. Description of the Related Art
A rack-and-pinion type steering apparatus is frequently used in automobiles. Rotation of the steering wheel by the driver is transmitted to a pinion meshed with a rack shaft. The rack shaft is connected to a steering rod for controlling the front wheels. The driver can therefore control the direction the automobile moves by rotating the steering wheel. The steering mechanism as described above is well known in the related art so further explanation is omitted here.
In the prior art, rack teeth have been obtained by gear cutting from a solid rod work piece. In the original steel bar for the workpiece, longitudinal metal flow is formed along the rolling direction. An anisotropy in strength between the direction parallel to the metal flow and the direction perpendicular to the metal flow occurs. The metal flow is cut off when the portions of the work piece are removed in the gear cutting process, therefore the rack teeth have low strength. So in order to obtain sufficient strength, the rack shaft has to be made larger, therefore, becomes heavier.
For reducing the weight of the rack shaft, a method is disclosed in Japanese Laid-Open Patent No. Hei 5-345231 for manufacturing a hollow rack shaft. In this method, a hollow rod material having sufficient thickness for gear cutting is prepared, and a portion of the wall of the hollow work piece is formed into a flat section by plastic deformation. Then, rack teeth are cut on the flat section. A cross section of the rack teeth produced by this manufacturing method is shown in FIG. 12. The teeth are formed by removing a portion of the material R, so that the metal flow F is cut off. In the above method (FIG. 12), it is not different from the prior art about an anisotropy in strength between the direction b parallel to the metal flow and the direction a (perpendicular to the metal flow), therefore the rack shaft must be large and heavy.
Another method for manufacturing rack shaft is disclosed in Japanese Laid-Open Patent No. Sho 58-218339 (Japanese Patent Publication No. Hei 4-28582). In this method, a hollow rod material is prepared and a flat section is formed on one portion of the surface by plastic deformation. Under the state that a mandrel is inserted into the hollow section, a die having teeth corresponding to the rack teeth is pressed onto the flat section to form the rack teeth. As the rack teeth are formed by plastic deformation in this method, the above-mentioned problem, i.e. cut of the metal flow F, does not occur. However, in this method, the work piece whose thickness (volume) is equal to the total volume of the rack teeth and the rack teeth bottom portion is required, thus the thickness except the rack teeth portion is unnecessarily thick. This causes a problem that the rack shaft could not be made sufficiently light.
Also, as shown in FIG. 13, the inner surface IS on the back side of the rack teeth is formed in a flat surface so that the metal flow F is sparse at sections RA of each tooth and are dense at the midsection of teeth, in other words, in the vicinity of the tooth bottom BA. A uniform strength could not be obtained since the density of the metal flows F is different at each portion. A cross sectional view of the same rack teeth is shown in FIG. 5. This figure shows the distribution of equi-stress lines when a load F is applied to one point f on the tooth surface (meshing surface with pinion). The equi-stress lines 4 in FIG. 5 appear densely on the front and back in the vicinity of the teeth bottom. The equi-stress lines 4 do not appear at other points, therefore it shows few changes in the stress. The portions where the equi-stress lines 4 are sparse hardly contribute to bear the load and are not essential for the rack shaft. These non-essential sections prevent the weight of the rack shaft from being reduced.
A manufacturing method for a hollow rack shaft conceived by Okubo, one of the inventors of the present invention, is disclosed in Japanese Patent Laid-Open 11-180318. In this method, a rectangular plate work piece is bent to a gutter-shaped member, and a rack teeth row formed on the flat center portion by plastic deformation, and the two leg portions of the gutter-shaped member are bent to butt against each other to produce a hollow rack shaft. In this method, the back surface of the rack teeth (surface forming the inner side of the hollow section in the future) IS, is formed with a series of wave-like concave-convex section corresponding to the rack tooth surface, unnecessary material do not remain and this method greatly contributes to reducing the weight of a rack shaft.
However, this Japanese Laid-Open Patent does not disclose how to optimize the uneven shape of the rack tooth back surface in order to prevent the stress concentration, nor what metal flow is optimum in the vicinity of the rack teeth.
An object of the present invention is to provide a hollow rack shaft with high strength and long service life by plastic forming a row of rack teeth without cutting metal flow at outer and inner surfaces of the hollow rack shaft.
Another object of the present invention is to provide a hollow rack shaft formed with a series of wave-like concave-convex sections on the inner surface. The stress concentration on the inner surface can be avoided and the stress is distributed equally. Therefore a hollow rack shaft with high strength and light weight can be provided. The hollow rack shaft in this invention is formed from a plate workpiece.
A hollow rack shaft according to the present invention is obtained by welding a gutter-shaped semi-circular (semi-tubular) upper half and a semi-circular lower half. The semi-circular upper half is formed with a row of rack teeth and is formed from a plate shaped work piece.
Inner surface of the hollow rack shaft according to the present invention has a series of wave-like concave-convex sections substantially corresponding to the rack tooth surface and the unevenness is provided with tooth wall thickness for uniformly distributing the tooth stress when a load is applied to the shaft. The hollow rack shaft of the present invention is defined by specified dimensions of portions on the cross section of the rack teeth row.
Other objects and advantages besides those described above, shall be apparent to those skilled in the art from the description of the preferred embodiments of th invention which follows. In the description, reference is made to the accompanying drawings, which form a part thereof, and which illustrate a working example of the invention. Such examples however, do not exhaustively cover the various embodiments of the invention, and reference is therefore made to the claims which follow the description for determining the scope of the invention.